Refining titanium alloys



nitcd States Patent REFINING TITANIUM ALLOYS William W. Gullett, College Park, Md., assignor to Chicago Development Corporation, Riverdale, Md.

No Drawing. Application March 23, 1956 Serial No. 573,336

5 Claims. (Cl. 204-64) This invention relates to the electrorefining of titanium alloys. It relates in particular to refining by anodic solution of titanium in halide electrolytes leaving more noble impurities and oxygen in the anodic residue.

The anodic solution and cathodic deposition of titanium is known in the art. The known procedures utilize electrolytes of alkalinous metal chlorides containing lower chlorides of titanium. When the titanium used as an anode contains substantial amounts of alloying elements, such as iron, chromium, manganese, and copper and oxygen, the titanium anode may become passive even at moderate current densities, and the more noble impurities will dissolve and be carried into the cathodically formed titanium. By substantial quantities of alloying elements I mean to include these elements in excess of the incidental amounts in commercial titanium, namely, .25% O .5% Fe, .25% Mn, .20% Cr, and .10% Cu. By moderate current densities I mean to include those above 100 amperes/sq. ft. and up to 3000 amperes/sq. ft. The expression alkalinous metal as used herein is intended to mean the members of the group consisting of alkali metals and alkaline earth metals.

I have found that the solution of the objectionable alloying elements from titanium anodes cannot be controlled to any substantial degree by regulation of the conventional parameters, current density, concentration or valence of the dissolved titanium chloride.

In order to insure that the titanium dissolves preferentially to the more noble impurities over a wide range of anodic current densities, I have found that the electrolyte of fused alkalinous chloride or bromide must contain not only a lower chloride of titanium but in addition a significant amount of dissolved alkalinous metal but not enough to form titanium metal by chemical reaction at the anode. The electrolyte in the anode zone must contain this alkalinous metal so that titanium will dissolve anodically as TiCl and in this way be less noble than the metallic alloying elements in the anode. The dissolved TiCl diffuses away from the anode and is reduced to titanium in the cathode zone. The means or form of such reduction are known in the art, and are not a part of my invention which is concerned with anode refining.

The concentration of alkalinous metal in the anode zone may be very small, but must be maintained throughout the refining operation. This may be done by the direct addition of alkalinous metal to the electrolyte either continuously or at intervals. This alkalinous metal may be conveniently contained in the make-up electrolyte to replace that removed with the titanium formed in the cathode zone. Such make-up electrolyte may be made in known ways, but conveniently by reacting sodium with TiCl at about 450 C. so as to form TiCl and sodium dissolved in sodium chloride. The amount of sodium which may be dissolved in a fused solution of lower titanium chloride without forming titanium metal is, usually, higher at higher concentrations of dissolved titanium. By using a relatively high concentration of soluble titanium in the electrolyte, say 5%, .2% sodium may be dissolved in an electrolyte basically composed of fused sodium chloride. This sodium tends to be chlorinated at the anode and renewed at the cathode. Diffusion of sodium from cathode to anode in reverse direction to the movement of soluble titanium is therefore a preferred condition of my process. This is assured by cell geometry and current density.

One preferred arrangement for the practice of my invention is a large surface anode, made up e. g. of alloy fragments and a small cathode in a single compartment cell of considerable volume. With this arrangement anodic current densities from .50-3000 amperes/sq. ft. may be used and cathode current densities from -5000 amperes/sq. ft. The open circuit voltage of this cell is the reverse of closed circuit voltage since sodium is more concentrated around the anode than cathode.

In other forms of my invention a secondary cathode may be used to provide sodium near the anode without forming titanium at this point.

The original alkalinous metal containing electrolyte may be produced electrolytically and in other known Ways.

Since the determination of dissolved alkalinous metal in the presence of titanium dihalides involves some unusual procedures it is noted here that I prefer to make this determination optically by examining the solidified electrolyte. The presence of alkalinous metal is demonstrated by a strong absorption band. The strength of this absorption band may be used to determine the amount of alkalinous metal since all alkalinous metals; have characteristic absorption bands.

The concentration of alkalinous metals can also be determined electrochemically. The potential of an inert (e. g. iron) electrode immersed inthe electrolyte assumes a potential proportional to the concentration of alkalinous metal.

A strictly chemical method may be used to determine the alkalinous metal dissolved in the electrolyte containing titanium dichloride. In this method, the salt is dissolved in a strong, slightly acid, solution of titanium trichloride and the evolved hydrogen measured after a considerable period. Every mol of titanium dichloride ultimately forms by reaction with acid one equivalent of hydrogen. The excess of hydrogen evolved over that corresponding to the amount of titanium dichloride represents the equivalent of alkalinous metal present. This method is subjected to some uncertainty due to possible disproportionation of the electrolyte before or during the analytical procedure.

When the titanium alloy to be refined contains substantial oxygen, e. g. ;5-5%, I add finely divided TiO to prevent solution of the oxygen in the electrolyte and cause it to remain in the anode residue.

Having now described my invention in its more general features, I will now illustrate it by particular examples.

Example I In this example I used an electrolytic cell of steel, 12 inches in diameter and 3 feet deep. This cell was provided with an argon atmosphere and an electrolyte of sodium chloride to which has been added 10% TiCl and 0.2% sodium. The temperature of operation is 850 C.

A 2 inch rod of this immersed 18 inches in the electrolyte formed the anode while a inch iron rod immersed 2 inches formed the cathode. A direct current of 100 amperes at an impressed voltage of .5 volt was passed from anode to cathode. The instantaneous open circuit of the cell was .2 volt in the reverse direction to the im- Patented Dec. 24, 1957,

pressed voltage, Titanium was formed in crystalline form adherent to the cathode. After washing with dilute acid to remove adhering salt, this titanium analyzed:

Percent Si trace Percent Fe .005 Percent Mn .002 Percent Cr .007 Percent .03 Balance Ti.

Example 11 In this example I used an electrolytic cell similar to that of Example I. I placed an anode of the following composition in the center of the cell:

Percent Cu 10.0 Percent 0 3.2 Percent Fe 6.1 Percent Si 1.0

Balance substantially Ti.

The electrolysis was carried out at a temperature of 850 C. The wall served as a cathode. The electrolyte consisted of NaCl 24.27%, BaCl 72.82%, titanium as chloride 0.97% and barium as dissolved metallic barium 1.94%. The concentration of barium in the anode area was maintained by the use of a perforated secondary electrode between the anode and cathode. This secondary electrode is maintained at a potential, within the range 0.l0.5 volt, with respect to the anode, which maintained the barium content of the electrolyte in the anode zone. The anode current density was 2000 amperes/sq. ft. While the cathode current density was 250 amperes/ sq. ft. The impressed voltage was .7 volt and the instantaneous open circuit voltage was .1 volt in the opposite direction. Titanium was deposited in the cathode Zone in crystalline form having the following analysis after washing with dilute hydrochloric acid:

Percent Cu .001 Percent 0 .02 Percent Fe .007 Percent Si .002

Example III I proceeded as in Example I except that the electrolyte was sodium bromide to which had been added 3 percent titanium as Til3r and 0.2 percent sodium. The operating conditions and results were the same as in Example 1.

Example IV I proceeded as in Example I using as an anode a titanium-oxygen alloy containing 5 percent oxygen. From time to time I added to the electrolyte in the anode zone finely divided TiO in small amount. The analysis of the product was .015 percent oxygen and only traces of other impurities, balance titanium.

I claim:

1. As a new composition of matter a single phase liquid consisting essentially of at least one chloride selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, in molten state, and dissolved therein from 0.255.0% soluble titanium as a lower chloride and from 0.l2.0% of a metal selected from the group consisting of alkali and alkaline earth metals.

2. The method of electrorefining a titanium alloy containing oxygen and an alloying metal more noble than titanium, which comprises making the comminuted alloy an anode in an electrolytic cell having a cathode and an electrolyte consisting essentially of at least one chloride selected from the group consisting of alkali metal chlorides and alkaline earth metal chlorides, in molten state, and dissolved therein 0.25-5.0% soluble titanium as lower chloride and 0.l2.0% of a metal selected from the group consisting of alkali and alkaline earth metals, and passing a direct current from anode to cathode at an anode current density of .503000 amperes per square foot and a cathode current density of 5000 amperes per square foot whereby to dissolve titanium as TiCl only and to leave more noble metals in the anode residue.

3. The method of claim 2, further characterized by the addition of finely divided TiO to the electrolyte in the anode zone whereby to minimize the transport of oxygen from the anode to the cathode.

4. In a process for ele-ctrorefining titanium alloys containing oxygen in solid solution and at least one metal more noble than titanium, the steps of passing a direct current between the alloy as anode to an inert cathode in an electrolytic cell having an electrolyte composed essentially of at least one halide of the group consisting of chlorides and bromides of alkali and alkaline earth metals and dissolved therein a metal selected from the group consisting of alkali and alkaline earth metals in an amount from about 0.1% to about 2.0% and a lower titanium halide selected from the group consisting of tially throughout the electrolysis a significant amount of dissolved alkalinous metal such amount of dissolved alkalinous metal being within the range 0.l2.0% and insufiicient to form titanium by chemical reaction with the lower titanium halide whereby to dissolve the titanium from the anode and to form refined titanium at the cathode substantially free from the oxygen and more noble impurities in the anode.

5. In the electrorefining of a titanium alloy containing oxygen in solid solution and an alloying metal more noble than titanium, by a procedure involving making such alloy an anode in an electrolytic cell having an inert solid conductive metal cathode and a molten salt electrolyte comprising a halide selected from the group consisting of chlorides and bromides of alkali and alkaline earth metals, and passing a direct current from anode to cathode through said electrolyte, the improvement which consists in depressing dissolution of oxygen and alloying metal from said anode whilst dissolving titanium there from by initially establishing in that zone of the electrolyte which is adjacent the anode, and thereafter dur, ing the electrolysis maintaining in said zone, a small but significant concentration of dissolved metal selected from the group consisting of alkali and alkaline earth metals within the range 0.12.0%.

References Cited in the file of this patent UNITED STATES PATENTS Steinberg et al. Apr. 26, 1955 Schultz et al. Feb. 14, 1956 

1. AS A NEW COMPOSITION OF MATTER A SINGLE LIQUID CONSISTING ESSENTIALY OF AT LEAST ONE CHLORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL CHLORIDES AND ALKALINE EARTH METAL CHLORIDES, IN MOLTEN STATE, AND DISSOLVED THEREIN FROM 0.25-5.0% SOLUBLE TITANIUM AS A LOWER CHLORIDE THERIN FROM 0.1-2.0% OF A METAL SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALALINE EARTH METALS.
 2. THE METHOD OF ELECTROREEFINING A TITANIUM ALLOY CONTAINING OXYGEN AND AN ALLOYING METAL MORE NOBLE THAN TITANIUM, WHICH COMPRISES MAKING THE COMMINUTED ALLOY AN ANODE IN AN ELECTROLYTIC CELL HAVING A CATHODE AND AN ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST ONE CHLORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL CHLO-RIDES AND ALKALINE EARTH METAL CHLORIDES, IN MOLTEN STATE, AND DISSOLVED THEREIN 0.25W SOLUBLE TITANIUM AS LOWER CHLORIDE AND 0.1-2.0% OF A METAL SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METALS. AND PASSING A DIECT CURRENT FROM ANODE TO CATHODE AT AN ANODE CURRENT DENISTY OF .50-3000 AMPERES PER SQUARE FOOT AND A CATHODE CURRENT DENSITY OF 100-5000 AMPERES PER SQUARE FOOT WHEREBY TO DISSOLVE TITANIUM AT TICL2 ONLY AND TO LEAVE MORE NOBLE METALS IN THE ANODE RESIDUE. 